U.S. patent number 4,007,129 [Application Number 05/525,799] was granted by the patent office on 1977-02-08 for partial combustion process for manufacturing a purified gas containing hydrogen and carbon monoxide.
This patent grant is currently assigned to Shell Oil Company. Invention is credited to Bernardus H. Mink, Jaap E. Naber.
United States Patent |
4,007,129 |
Naber , et al. |
February 8, 1977 |
Partial combustion process for manufacturing a purified gas
containing hydrogen and carbon monoxide
Abstract
An improved process for manufacture of a purified gas containing
hydrogen and carbon monoxide is described wherein a fuel is
partially combusted in a reactor to produce a crude gas product
containing soot, ash and contaminating gases such as HCN, H.sub.2 S
and COS, the crude gas product is subsequently cooled in a waste
heat boiler and the cooled gas is washed in a scrubber to remove
the entrained soot particles. In this improved process, removal of
soot, ash and gaseous contaminants from the cooled crude gas
product is facilitated by washing the cooled gas in a scrubber with
an aqueous salt solution containing at least 10% by weight of an
alkaline water-soluble salt, which salt solution is subsequently
regenerated by removal of absorbed gases and suspended solids (soot
and ash) and recycled to the scrubber.
Inventors: |
Naber; Jaap E. (Amsterdam,
NL), Mink; Bernardus H. (The Hague, NL) |
Assignee: |
Shell Oil Company (Houston,
TX)
|
Family
ID: |
10472329 |
Appl.
No.: |
05/525,799 |
Filed: |
November 21, 1974 |
Foreign Application Priority Data
|
|
|
|
|
Nov 27, 1973 [UK] |
|
|
54876/73 |
|
Current U.S.
Class: |
252/373;
252/375 |
Current CPC
Class: |
B01D
53/14 (20130101); C01B 3/36 (20130101); C01B
3/50 (20130101); C01B 2203/0465 (20130101); C01B
2203/0475 (20130101); C01B 2203/0485 (20130101); Y02P
20/129 (20151101) |
Current International
Class: |
B01D
53/14 (20060101); C01B 3/50 (20060101); C01B
3/00 (20060101); C01B 3/36 (20060101); C01B
002/02 (); C01B 002/14 () |
Field of
Search: |
;252/373 ;48/215,197R
;423/223 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd. Ed., vol. 4,
360-361..
|
Primary Examiner: Mars; Howard T.
Attorney, Agent or Firm: Vance; Dean F.
Claims
What is claimed is:
1. In a process for manufacturing a hydrogen and carbon
monoxide-containing gas by partially combusting a hydrocarbonaceous
fuel in a reactor, cooling the crude gas obtained in a waste heat
boiler and washing the cooled gas in a scrubber to remove entrained
soot particles, the improvement which comprises; washing the crude,
cooled gas in a scrubber with an aqueous salt solution containing
at least 10%w of an alkaline water-soluble salt, said alkaline
water-soluble salt being selected from the group consisting of
sodium carbonate, potassium carbonate, sodium phosphate, potassium
phosphate and mixtures thereof, which salt solution is subsequently
regenerated by removal of absorbed gases and suspended soot and
recycled to the scrubber and wherein said gas leaves the scrubber
at a temperature of at least about 100.degree. C.
2. The process according to claim 1, wherein the said fuel is
partially combusted at elevated pressure in the reactor, the crude
gas obtained thereby is cooled at substantially the same pressure
in the waste heat boiler and the cooled gas is washed at
substantially the same pressure in the scrubber, the temperature of
the salt solution in the scrubber being below its boiling point at
the prevailing pressure.
3. The process according to claim 1, wherein the temperature of the
cooled gas entering the scrubber is above 150.degree. C.
4. The process according to claim 3, wherein the temperature of the
cooled gas entering the scrubber is above 250.degree. C.
5. The process according to claim 4, wherein the temperature of the
washed gas leaving the scrubber is between about 150.degree. and
about 100.degree. C.
6. The process according to claim 5, wherein the temperature of the
salt solution when entering the scrubber is below 100.degree.
C.
7. The process according to claim 6, wherein the temperature of the
salt solution leaving the scrubber is between about 200.degree. and
about 150.degree. C.
8. The process according to claim 1, wherein the salt solution
contains 30-40%w of the salt.
9. The process according to claim 8, wherein the solution contains
sodium phosphate.
10. The process according to claim 8, wherein the solution contains
sodium carbonate.
11. The process according to claim 1, wherein the absorbed gases in
the aqueous salt solution effluent from the scrubber are removed in
a stripper by stripping the solution with steam.
12. The process according to claim 11, wherein said regeneration is
carried out by stripping with low-pressure steam of between
100.degree. and 150.degree. C.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a process for manufacturing a
hydrogen and carbon monoxide containing gas by partially combusting
a fuel in a reactor, cooling the gas obtained in a waste heat
boiler and washing the cooled gas in a scrubber to remove entrained
soot particles. More particularly, the invention is directed to an
improvement in the above-mentioned conventional process wherein the
cooled gas washing step is carried out in a manner such that soot
separation occurs more readily and subsequent processing of the gas
to remove conventional gaseous contaminants is substantially
avoided.
By the partial, i.e., sub-stoichiometric combustion of gaseous
liquid or solid hydrocarbonaceous fuel, a gas is obtained that
predominantly consists of hydrogen and carbon monoxide. The fuel
can be combusted with oxygen and this process may yield a synthesis
gas for manufacturing, e.g., methanol or technically pure hydrogen.
The carbon monoxide can be converted catalytically with steam to
produce a gas containing more hydrogen. If air is used to support
the combustion, the gas will contain nitrogen as well and may, for
example, be used for manufacturing ammonia or for generating
electricity. In the latter case the partial combustion is carried
out at elevated pressure and the gas so obtained is usually first
expanded through a gas turbine. In other instances, the partial
combustion may be carried out at elevated pressures as well.
Generally, the gas that is obtained by partial combustion contains
an amount of soot, ash and contaminating gases such as HCN, H.sub.2
S, COS, CS.sub.2 and CO.sub.2.
Depending on the type of fuel and on the operation conditions,
amounts of soot of up to 3 or 4%w may be present in the crude gas
product emanating from the partial combustion reactor. The soot
particles are, in general, very small and cannot easily be removed
from the crude gas at the moment the latter leaves the partial
combustion reactor due to the high temperature of the gas.
Normally, this temperature is higher than 1000.degree. C.
The gas is typically passed from the reactor into a special waste
heat boiler, which is designed to withstand the elevated pressures
at which the gas is generated and to avoid becoming clogged by the
soot contained therein. In a waste heat boiler with helically
coiled tubes, for example, the soot is not deposited on the walls
of the tubes. High-pressure steam can be generated in the waste
heat boiler and the gas may, for example, be cooled to below
400.degree. C.
After this cooling step the soot and other contaminants can be
separated from the gas. According to one conventional procedure,
the soot particles may, for example, be removed by "quenching" the
gas with water in a scrubber in which the soot is washed out and an
aqueous soot slurry is obtained. The ash is also taken up by the
water. The temperature of the gas is further reduced by this soot
removal step.
Subsequently the gaseous contaminants may be absorbed from the gas
in an absorber column with the aid of an absorber liquid.
Purification of the crude partial combustion gas by this two-step
procedure is considered advantageous because contamination of the
absorber liquid for gaseous contaminants with soot is avoided by
this route.
However, separation of solid and gaseous contaminants in two
separate steps is expensive and heat losses unavoidably occur in
the process because of the temperature constraints inherent in the
gaseous absorption step. Furthermore, the density of the soot
obtained in partial combustion is typically rather close to that of
water and as a result it is difficult to obtain a good separation
of soot from water using conventional separation techniques.
The instant invention provides a process by which the soot and
gaseous contaminant removal steps are combined in a single step and
the problems associated with separate operation of each step are
substantially eliminated.
SUMMARY OF THE INVENTION
Accordingly, the instant invention provides an improved process for
manufacturing a hydrogen and carbon monoxide-containing gas by
partially combusting a fuel in a reactor, cooling the crude gas
obtained in a waste heat boiler and washing the cooled crude gas in
a scrubber to remove entrained soot particles, characterized by the
improvement which comprises washing the crude, cooled gas in a
scrubber with an aqueous salt solution containing at least 10%w of
an alkaline water-soluble salt, which salt solution is subsequently
regenerated by removal of absorbed gases and suspended soot and
recycled to the scrubber.
When the cooled gas is washed with the salt solution, the
temperature in the scrubber may be higher than in the case of the
abovementioned two-step process, because of the solution's capacity
for removal of acid gases at temperatures much higher than those
used in the conventional process. Accordingly, the temperature of
the gas leaving the scrubber may also be higher than in the
conventional process. This is in many cases an advantage; for
example, when the gas must be subjected to further treatments,
e.g., the catalytic conversion of carbon monoxide at higher
temperature or when the gas is subsequently expanded through a gas
turbine. Moreover, when the temperature of the purified gas leaving
the scrubber is higher, this means that less steam has been
condensed and so a smaller amount of contaminated condensate is
obtained in the process. A high percentage of steam in the purified
gas can be attractive for a variety of reasons; for example, when a
catalytic conversion of carbon monoxide is to be carried out. Also,
the energy left in the steam can be recovered when expanding the
gas through a gas turbine.
As mentioned, the gas is washed with a salt solution. The
subsequent separation of the soot from the salt solution will be
easier than in the case wherein the gas is washed with water,
because of the higher density of the solution versus that of the
usual washing water. This higher density makes flotation and
similar soot separation techniques easier.
By washing the gas with an alkaline solution, further contaminants
such as ash, HCN, COS, H.sub.2 S and CO.sub.2 are effectively
removed from the gas, so that a small amount of washing liquid is
needed and the gas is cooled less deeply than in the two-step
removal process.
The gas containing hydrogen and carbon monoxide is often produced
at high pressure, for example, because it is needed at such a
pressure or because energy is to be recovered therefrom by
expansion through a gas turbine. It is preferred to operate the
waste heat boiler and scrubber at substantially the same pressure
as the reactor. In the two-step process according to the prior art,
wherein soot and ash are removed with quenching water in a scrubber
and contaminating gases with an absorber liquid in an absorber, the
gas has to be cooled to the relatively low temperature at which the
absorber is operated. This temperature is lower than the
temperature at which the purified gas is available in the instant
process.
A further advantage of the integration of the removal of soot and
contaminating gases is the fact that the soot particles consist of
highly active carbon that can absorb an additional quantity of
contaminating gases so that less salt solution is needed than would
be expected for gaseous absorption with the salts in question in
the absence of soot.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Partial combustion according to the present invention can be
suitably carried out using air, oxygen enriched air or oxygen as
the oxidant source (oxygen-containing gas). In any case it is
contemplated that the reaction will be conducted in a conventional
manner under conditions of temperature and pressure such that the
reaction is self-supporting. Accordingly, the reaction temperature,
broadly stated, will range from about 700.degree. to about
2000.degree. C with the reaction pressures ranging from atmospheric
up to about 600 psig. Within this broad range the reaction
temperature and pressure are preferably 900.degree.-1400.degree. C
and atmospheric to 30 psig, respectively, when air is used as the
oxidant source with some what higher temperatures and pressures,
e.g., 1100.degree.-1700.degree. C and atmospheric to 600 psig,
being employed when oxygen is used as the source of
oxygen-containing gas.
The hydrocarbonaceous fuels which are suitable for use in the
process of the invention include any hydrocarbon-containing feed
material which is capable of being made sufficiently fluid, i.e.,
by preheating and/or the addition of steam via conventional means
to be injected by conventional nozzle devices into the partial
combustion reactor. Natural gas or normally gaseous hydrocarbons
such as C.sub.2-4 saturated and olefinic hydrocarbons because of
their availability and ease of handling are particularly suitable,
however, heavier hydrocarbon fractions including gasoline,
kerosene, naphtha, distillates, gas oils and residual oils can also
be used as feed materials. In addition, such materials as coal
distillation gas and the effluent from liquefaction or gasification
of coal can also be suitably employed.
According to the present invention there is a special incentive, as
mentioned above, to operate the scrubber at as high a pressure as
possible. Accordingly, a preferred embodiment of the invention is
to partially combust the fuel at elevated pressure in the reactor,
to cool the gas obtained at substantially the same pressure in the
waste heat boiler and to wash the gas at substantially the same
pressure in the scrubber, the temperature of the solution in the
scrubber being below its boiling point at the prevailing pressure.
In this embodiment of the invention the temperature of the gas
leaving the scrubber will be as high as possible.
Preferably, the gas is washed at such a pressure and temperature
that substantially no steam is condensed therefrom during washing.
An advantage thereof is that no additional amount of contaminated
water is produced, so that no bleed stream disposal problems occur.
Optionally, water is supplied to the gas prior to the washing in
order to further saturate the gas; however, this is done not beyond
the extent at which condensation starts. The latter measure
contemplates that no water evaporates from the salt solution that
is used in the scrubber, whereby the water balance for the process
is such that no water from outside sources is needed.
The salt solution is preferably regenerated at substantially lower
pressure than the pressure prevailing in the scrubber when the gas
is washed at high pressure. This is desirable, since the removal of
the adsorbed gases is facilitated by the resulting pressure
drop.
The temperature of the cooled gas entering the said scrubber
preferably lies above 150.degree. C. In general, as mentioned
above, the integrated removal of solid and gaseous contaminants
from the gas leaving the waste heat boiler is more advantageous at
higher temperatures. A temperature of the gas entering the scrubber
of at least 250.degree. C is preferred according to the
invention.
The temperature of the washed gas leaving the scrubber in the
process according to the invention, will in general lie below
150.degree. C. The ultimate temperature depends on the temperature
at which the gas enters the scrubber, on the dew point of the
entering gases, on the amount of salt solution needed to remove the
amount of soot present, etc. Under certain conditions the
temperature at which the purified gas leaves the scrubber will lie
above 150.degree. C. Preferably, this temperature lies at least
about 100.degree. C.
Although the regeneration of the salt solution that is loaded with
solid and gaseous contaminants will at least partly take place at a
high temperature, it is preferred to cool the regenerated salt
solution before it is recycled to the scrubber. According to a
preferred embodiment of the invention, the temperature of the salt
solution when entering the scrubber lies below 100.degree. C.
Generally, according to the invention, the temperature of the salt
solution when leaving the scrubber lies below 200.degree. C.
Preferably, this temperature would be above 150.degree. C in order
to ensure that the temperature of the purified gas leaving the
scrubber is as high as possible.
The concentration of the alkaline salt solution should be at least
10%w according to the invention, in order to attain the desired
effects. The concentration that can be used is limited by a
diversity of factors, including the solubility of the salt under
the operating conditions. In any case, care should be taken that
the salt does not crystallize in the scrubber when operation is
interrupted for some reason and the solution is permitted to cool
down.
A preferred range of salt concentrations according to the invention
is 30-40%w. At such concentrations, an attractive increase both in
absorption capacity and in specific gravity of the solution is
obtained, as compared to quench water, while the concentrated
solution is still manageable.
Preferred salts include sodium and potassium carbonate, sodium and
potassium phosphate and mixtures thereof. Solutions containing
these salts have a fairly high capacity for absorbing the gaseous
contaminants mentioned earlier on and the salts have a good
solubility and thermal stability under the conditions prevailing
during operation. It will be clear that other salts or combinations
of salts may be used as well.
An attractive way of removing the absorbed gases from the load salt
solution, which inter alia is preferably carried out before removal
of the ash and soot, is by stripping the solution with steam in a
stripper. In this regeneration procedure it is possible to use
exactly the amount of steam, having an appropriately high
temperature, needed to vaporize an amount of water equal to the
amount of steam that condenses from the gas when it passed the
scrubber. In this way the amount of recirculating salt solution and
the concentration thereof are kept constant.
Stripping is preferably carried out with low-pressure steam of
between 100.degree. and 150.degree. C. The pressure of the
stripping steam can advantageously be of the same order as the
pressure at which the solution is subsequently treated to remove
soot and ash. This pressure will generally be about
atmospheric.
Before, during or after removal of the absorbed gases from the
solution the soot, and optionally also the ash, have to be removed
from the salt solution.
According to a preferred embodiment of the invention the soot is
removed from the salt solution in a pelletizer by pelletizing the
soot particles and separating the pellets obtained from the salt
solution. Pelletization, i.e., making pellets from the soot
particles while they are suspended in the salt solution or float on
top of it, can be carried out according to a variety of per se
known methods.
Pelletization may be carried out, for example, by adding binder to
the soot-containing salt solution while stirring vigorously and by
then separating the agglomerates of soot and binder obtained from
the salt solution. It is possible to carry out this pelletization
in such a way that the ash is at least partly taken up by the
pellets or agglomerates and so removed from the solution.
Pelletization may be carried out with all kinds of binder, such as
binders based on hydrocarbon oil products. Examples of such binders
are naphtha, gas oil and light fuel oil.
A suitable way to remove the soot from the salt solution is by
taking up the soot particles, either as such or after
pelletization, in a hydrocarbon phase. In this way separation of
the soot particles or pellets from the salt solution is
facilitated, especially because of the large difference in specific
gravity between the concentrated salt solution and the hydrocarbon
phase. A suitable hydrocarbon phase is a heavy hydrocarbon oil. The
use of a heavy hydrocarbon oil makes it possible for example, to
take up the soot particles directly in the fuel oil destined for
the partial combustion, and so to recycle the soot.
According to another embodiment of the invention the soot particles
are pelletized with naphtha and the agglomerates obtained, after
having been separated from the salt solution, are taken up in a hot
fuel oil. Accordingly, the naphtha is stripped off. The naphtha
thus recovered is recycled to the pelletizer, while the fuel oil
with soot is used as part of the fuel for the partial
combustion.
When a sulfur-containing fuel is partially combusted, the hydrogen
sulfide formed can be absorbed from the obtained gas by the salt
solution and ultimately be removed therefrom by stripping. During
combustion, HCN, COS, CS.sub.2 and/or CO.sub.2 may also have been
formed in the reactor and these gaseous contaminants can also be
removed and/or converted in the scrubber from the cooled gas which
has left the waste heat boiler by absorption in the salt solution
and ultimate removal therefrom by stripping.
According to the invention a hydrocarbonaceous fuel that contains
traces of metal compounds can be subjected to the partial
combustion. The ash formed is then taken up by the salt solution in
the scrubber. The ash can be separated as insoluble alkaline
compounds from the salt solution during regeneration thereof. When
the soot is pelletized, the ash can be separated together with the
soot. Preferably, when the washing of the gas with the salt
solution in the scrubber is carried out to such an extent that
steam condenses from the gas, the build-up of ash in the salt
solution is prevented by withdrawal of a bleed stream before the
salt solution is recycled to the scrubber, the concentration of
salt in the salt solution being kept at a constant level. In this
case, it is not necessary to specially separate the ash from the
salt solution, provided that a certain permanent percentage of ash
is permitted in the recycling salt solution.
The fuel to be partially combusted may contain, for example,
compounds of vanadium, calcium, magnesium and other metals that are
frequently encountered in hydrocarbon fuels or solid fuels.
The invention is further illustrated by the following example
carried out according to the invention.
EXAMPLE
The apparatus used consists in its principal parts of an absorber
column containing two sections of Raschig rings, a soot slurry
flash tank, a pelletizer device and a stripper column.
A heavy fuel oil was partially combusted with oxygen in a reactor
at a pressure of 30 atm., whereby a gas was obtained that
predominantly consists of hydrogen and carbon monoxide and that
contains 2.3%w of soot and ash, 1.1%v H.sub.2 S, 180 ppm by volume
HCN and 210 ppm by volume COS. The temperature of this gas was
about 1400.degree. C when leaving the reactor and the gas was
cooled to about 250.degree. C by passing it through a waste heat
boiler, thereby raising steam of high pressure. Part of this steam
was passed to the stripper column for stripping the salt
solution.
The cooled gas was passed, into the absorber column; near the
bottom thereof, i.e., below the lower section of Raschig rings. In
between the two sections of Raschig rings a concentrated aqueous
solution of Na.sub.2 CO.sub.3, having a temperature of about
100.degree. C was introduced into the absorber column and passed
down through the lower section. Make-up water was introduced near
the top into the column and passed downward through both sections.
At the top the gas is withdrawn from the column. The absorber
column was operated at a pressure of about 30 atm.
At the bottom of the column the downwardly flowing water and
Na.sub.2 CO.sub.3 -solution, having been in countercurrent flow
contact with the gas, have gathered the soot and ash and acid gases
that were present in the gas to the extent that the washed gas
contains 15 ppm by weight of soot and ash, 19 ppm by volume H.sub.2
S, 9 ppm by volume HCN and 5 ppm by volume COS. A loaded salt
solution was withdrawn at the bottom of the column and passed to
the flash vessel at 170.degree. C. In the latter vessel 10% of the
water and part of the absorbed gases was separated from the salt
solution at reduced pressure of the salt solution.
From the flash vessel the salt solution was passed to the
pelletizer where soot and ash are removed by pelletizing them with
a heavy hydrocarbon oil binder into agglomerates.
The salt solution was then passed into the stripper column wherein
it is stripped with steam at substantially atmosphereic pressure.
The stripping steam and the gases from the flash vessel can be
passed to a Claus-plant to remove the sulfur compounds therefrom.
The regenerated salt solution was recycled to the absorber column
at 100.degree. C.
* * * * *